pgsql: Further fixes to the pg_get_expr() security fix in back branches

Further fixes to the pg_get_expr() security fix in back branches.

It now emerges that the JDBC driver expects to be able to use pg_get_expr()
on an output of a sub-SELECT.  So extend the check logic to be able to recurse
into a sub-SELECT to see if the argument is ultimately coming from an
appropriate column.  Per report from Thomas Kellerer.

Branch
------
REL8_0_STABLE

Details
-------
http://git.postgresql.org/gitweb?p=postgresql.git;a=commitdiff;h=46286d62092c2f97fdc68ff0ffa02739395340e9

Modified Files
--------------
src/backend/parser/parse_func.c |   80 +++++++++++++++++++++++++++------------
1 files changed, 56 insertions(+), 24 deletions(-)